Fluids flowing through pipelines or tubes often need to be sampled to determine the composition of the fluid being transported. For example, payments for oil or gas delivered from a pipeline may depend on various characteristics of the oil or gas transported through the line. It is desirable to be able to sample the fluid in the pipeline easily and repeatedly without opening the pipeline, which is often under pressure. Because the composition of the transported fluid often varies with time, it is particularly important that the samples are actually collected at various times as may be required.
Thus there is a need for a device that permits removal of samples from pipelines, tubes, and the like without opening them while providing confirmation that actual samples have been collected at the prescribed times and frequencies.
Some conventional sampling systems have two pistons in a common passageway, with one having a tubular shaft mounted coaxially on the solid shaft of the other. This passageway is a side passageway through which fluid is only intended to flow during sampling. This side passageway intersects a main flow passageway, which is being sampled. The piston closest to the main passageway moves out of the side passageway and into the main passageway to expose the space in the side passageway between the pistons to the fluid sample. The piston furthest from the main passageway allows flow into, but blocks flow through, the side passageway. Then both pistons are simultaneously drawn into the side passageway in a spaced relationship to capture a sample in the annular space in the side passageway between the pistons. The pistons are aligned with a lateral port in the side passageway and moved toward each other to force a sample of fluid out through the port. There is no continuous fluid flow through the side passageway, as the side passageway is either open only to the main passageway, open only to the outlet port, or open to neither outlet port nor main passageway.
Another prior sampling system has lateral inlets and outlets to a sampling passageway between the inlets and outlets. This has no pistons, the sampling being done by pressure responsive valve timers. This method results in a variable quantity of sample depending on pressure in the line being sampled, and depends on the pressure in the sample container being lower than that in the line being sampled. At least three separate pressure responsive valves and a lengthy sample line are required.